So the whole floor isn't sloping, there's just a ramp installed to marry the original floor to the botched floor height of the extension? Why not just remove the ramp and nail down another skin of ply? If you start needing 1/16 inches you'll have to shim & glue but you won't have to carry 1/16 across the whole room so it'll just be at the transition.
Won't you just end up with a step up into the newly raised area from somewhere else tho? Maybe that ramp is there because council won't approve a 'step' below a certain height as a trip hazard. Don't forget the baseboards.
I think floor leveling compound is intended for very thin skim coats on concrete slabs. You need a carpentry solution if you're ply on joists.